This disclosure relates to a developing device, an image forming apparatus having the same, and a toner concentration detecting method, and particularly to a technique for detecting a toner concentration of a developer contained in a developing device.
As a device for detecting a toner concentration of a developer contained in a developing device, a toner concentration sensor having an LC oscillator circuit is known. Such a toner concentration sensor treats a change in permeability changed by the toner concentration of the developer as a change in an oscillating frequency of the LC oscillator circuit, thereby detecting the toner concentration of the developer. In recent years, for the purpose of miniaturization or cost reduction, a toner concentration sensor using a planar coil for a coil constituting the LC oscillator circuit has also been released.
Here, in an image forming apparatus that conducts color printing, the toner concentration sensor needs to be installed for the developer of each of magenta, cyan, yellow, and black. When the planar coil is used for the coil constituting the LC oscillator circuit, the oscillating frequency of each toner concentration sensor becomes a high frequency of several MHz, and thus radiation noise is increased. For this reason, the radiation noise radiated from the multiple toner concentration sensors adds up, and the radiation noise radiated from the entire apparatus may be further increased.
With respect to the above problem, in the image forming apparatus having the multiple toner concentration sensors, a technique for suppressing the radiation noise radiated from the entire apparatus by controlling a switching operation of power and shifting timing to supply the power to each toner concentration sensor is known (Technique A). Further, a technique for providing an earth plate on a rear surface of a board for the toner concentration sensor and causing the earth plate to magnetically shield the radiation noise is known (Technique B).